The effect of shadowed areas on smothering risk in three commercial free-range layer poultry farms, Australia 2019–2022

Smothering, which results from birds crowding into densely packed groups (piling), is a multi-factorial problem influenced by bird behaviour, flock dynamics, and environmental factors. As part of a prospective cohort study to identify risk factors for smothering in Australian free-range layer poultry, flock managers consistently reported that outdoor smothering events were more likely to occur in shaded (as opposed to sunlit) areas. In this study we tested the hypothesis that outdoor smothering deaths were more common in shaded areas of the range. The data for this study were derived from a three-year prospective cohort study of 84 free-range layer flocks from three commercial free-range layer organisations in eastern Australia. Three-dimensional models of the physical structure of poultry sheds on each of the three organisations were constructed using a Geographic Information System. The time (in 60 minute increments) in which each cell of a regular 1 m by 1 m grid extending to 25 m around each shed’s boundary was in shadow was calculated using geometric shadow estimation procedures. The incidence rate of smothering deaths in shadow positive grid cells divided by the incidence rate of smothering deaths in shadow negative grid cells was calculated to provide an estimate of the unconditional strength of association between an area being in shadow and smothering death incidence rate. A mixed-effects Poisson regression model was used to control for the confounding effect of season. For all three organisations the number of smothering deaths per 10,000 shadow positive grid cell hours was 1.8 (95% CI 1.8 to 1.9) times that of the number of smothering deaths per 10,000 shadow negative grid cell hours. The strength of the association between shadowed areas and outdoor smothering risk varied across each of the three organisations that took part in this study and shed within organisation. Shadowed areas were positively associated with smothering risk for some sheds but not others. We conclude that measures to mitigate the effect of shaded areas on smothering risk should be developed on an individual shed level basis with provision of additional shade structures distributed broadly across the range to promote wider bird dispersal.